Wide attention has been given to metallocene catalysts for their ability to make ethylene polymers having relatively narrow molecular weight and comonomer distributions at excellent polymerization rates. Such narrow molecular weight and comonomer distributions contribute to improvements in clarity, toughness, and extractables levels in ethylene polymers having densities much below 0.95 g/cc. However, for some applications requiring processability, such as extrudability, these ethylene polymers can be deficient due to their narrow molecular weight distributions. For example, U.S. Pat. Nos. 5,420,220 and 5,324,800 disclose metallocene-made, linear low-density polyethylenes having characteristic narrow molecular weight and comonomer distributions, along with the associated limitations in processability.
Unfortunately, if the molecular weight distribution of an ethylene polymer is broadened in order to improve processability, the clarity and impact strength of the polymer diminish. In addition, extractables increase, especially for ethylene polymers having densities much below 0.93 g/cc. To improve the processability of an ethylene polymer while maintaining a narrow molecular weight distribution, long chain branching may be incorporated into the polymer. For instance, U.S. Pat. Nos. 5,272,236 and 5,278,272 and PCT Application No. WO94/07930 describe metallocene-made, very low-density and low-density polyethylene having long chain branch structures that are reported to have improved processability. However, long chain branch structures sometimes promote directional orientation during fabrication leading to an imbalance in mechanical properties and reduced impact and tear resistance. The clarity of fabricated articles such as blown film may also be less than optimum for long chain branched ethylene polymers even with narrow molecular weight and comonomer distributions.
Applicants have identified a family of ethylene polymers having a narrow molecular weight distribution and a narrow composition distribution compared to conventional linear-low density polyethylene made from Ziegler-Natta catalysts. However, surprisingly, the ethylene polymers also have a relatively narrow relaxation time distribution, defined by their Relaxation Spectrum Index (RSI), such that the processability of the ethylene polymers is comparative, at similar melt index, to conventional, linear-low density polyethylenes made with Ziegler-Natta catalysts, which have broader molecular weight distributions, and superior to many commercial metallocene-made polyethylenes.
Film articles made from the present ethylene polymers are characterized by superior clarity, higher impact strength (e.g., dart impact), and low extractables. Similarly, injection molded articles made from the present ethylene polymers have improved clarity and toughness (e.g., low temperature properties and ESCR) as characterized by higher impact strength compared to known linear low-density polyethylenes. The enhanced toughness associated with applicants' ethylene polymers offers potential for film downgauging and molded part "thin walling" while retaining sufficient strength. Further, the ethylene polymers provide superior performance in large-volume film applications, such as stretch, high clarity, and other packaging films. And due to the low extractables of the present ethylene polymers, film and molded articles made from them are attractive for use in the food packaging market.